1. Field of the Invention
The present invention relates to methods of and apparatus for inserting tubing into live wells, for example for gas and petroleum extraction.
2. Description of the Prior Art
A conventional method of completing or servicing a well is to keep the well controlled (dead) or in an over-balanced position using different types of fluids. In this way, a hydrostatic pressure is created by a solid column of fluid above a producing zone which is greater than the actual well pressure. This allows a work to be carried out on the well without any immediate danger or inconvenience.
Live well servicing or snubbing is primarily used in gas well applications because of fluid sensitive formations. Thus, it has been long proven that many gas well formations are very sensitive and can be damaged if foreign fluids are introduced and allowed to penetrate their producing zones. A common completion procedure in the oil industry today comprises perforating the zone, with no fluid in the well or in a under-balanced condition, immediately flowing back or unloading any fluid from the well, before the gas pressure equalizes in the casing and allows any fluid to be displaced or injected into the formation, and then snubbing-in a production string of tubing and associated tools.
Existing conventional snubbing techniques are dangerous because it is necessary to deal with the effect of the well pressure over the cross-sectional areas of the tubing and/or the tools which are being injected into the well through a series of blowout preventers.
The actual injecting of tubular goods into the well is achieved using a hydraulic jacking type of assembly with inverted or upside-down slips mounted in it to prevent the tubing from coming back or literally blowing out of the well. A general rule of thumb would be that 3000 PSI surface pressure well against the cross-sectional area of 23/8 inch production tubing will force 3000 feet of pipe weighing 4.7 lbs./foot out of the well if allowed to do so. In reverse, 3000 feet of tubing would have to be snubbed into the well to reach a point where it could be lowered into the well using its own weight (pipe heavy).
All operations are performed manually from within a small work platform that sits directly on top of the well. This has major potential consequences in the event of human error or mechanical failure. Because of the increased production potential that a well has when it has not been damaged by allowing any fluid to penetrate the zone, oil companies have unwillingly paid the brunt of inflated snubbing costs required to persuade personnel to work under these dangerous conditions.
Many companies are therefore actively looking for a viable, safe and economical alternative way of completing and servicing wells without the risk of formation damage and also without high cost and potential danger of snubbing. This is particularly true in the case of in critical sour (H2S) well applications.